The surprising Far-UV spectrum of the polar BY Camelopardalis^*

¹ UMR 8102, associée au CNRS et à l'Université Paris 7, LUTH, Observatoire de Paris, Section de Meudon, 92195 Meudon Cedex, France
² Université Paris 7 - Denis Diderot, 2 place Jussieu, 75005 Paris, France
³ Service d'Astrophysique, DSM/DAPNIA/SAp, CE Saclay, 91191 Gif-sur-Yvette Cedex, France
⁴ Universtäts-Sternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany
⁵ INAF, Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
⁶ UMR 7095, associée au CNRS et à l'Université Paris 6, Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
⁷ Braeside Observatory, Flagstaff, AZ 86002, USA
⁸ Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
⁹ Astrophysics Group, Planetary Science Institute, Tucson, AZ 85705, USA
¹⁰ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹¹ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany
¹² Department of Astronomy, University of Washington, Seattle, WA 98195, USA

Corresponding author: M. Mouchet, martine.mouchet@obspm.fr

Received: 2 October 2002
Accepted: 31 January 2003

Abstract

We report on the first far-UV observations of the asynchronous polar BY Cam made by the Far-Ultraviolet Spectroscopic Explorer (FUSE). The source is known to exhibit the most extreme NV/CIV emission resonance line ratio observed among polars. The FUSE observations reveal a OVI resonance line weaker than in the prototype of polars, AM Her, with the absence of a detectable narrow component. The OVI broad line is detected with an equivalent width of the same order as in AM Her, the blueward doublet component is clearly present but the redward component is strongly affected by H₂ absorption. The presence of a strong NIII line and weak CIII lines also confirms the peculiar CNO line flux. We compare the resonance CNO line intensities with the predictions of the CLOUDY plasma code coupled to a geometrical model of the accretion column. Varying the temperature and/or intensity of the ionising spectrum is unable to reproduce the observed broad line ratios. A solution is obtained by significantly altering the element abundances with a strong depletion of C, overabundance of N and a weak underabundance of O. This confirms previous suggestions of non-solar abundances which may result from redistribution in the accreted material following nova outbursts and/or the secondary nuclear evolution. A very significant H₂ absorption is observed in front of the source, a possible indication for either the existence of a dense interstellar cloud or of circumstellar material.